The goal of this project is to determine the crystal structure of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) from the green algae Chlamydomonas reinhardtii. This enzyme catalyzes the key step in oxygen to the substrate in a competing reaction. The ratio of these reactions, described by the specificity factor, has increased during phylogenetic divergence. Whereas Chlamydomonas Rubisco shares high sequence identity with the enzymes from spinach and tobacco (whose structures are known), its specificity factor is considerably lower. Elucidation of this structure will allow a description of the subtle details that determine this specificity.